A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus.) Other immersion liquids have been proposed, including water with solid particles (e.g., quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see for example U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, for example along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
Another solution which has been proposed is to provide the liquid supply system with a seal member which extends along at least a part of a boundary of the space between the final element of the projection system and the substrate table. Such a solution is illustrated in FIG. 4. The seal member is substantially stationary relative to the projection system in the XY plane though there may be some relative movement in the Z direction (in the direction of the optical axis). A seal is formed between the seal member and the surface of the substrate. In certain embodiments, the seal is a contactless seal such as a gas seal. Such as system with a gas seal is disclosed in European Patent Application No. 03252955.4, hereby incorporated in its entirety by reference.
In yet another solution as depicted in FIG. 5, a reservoir 10 forms a contactless seal to the substrate around the image field of the projection system so that liquid is confined to fill a space between the substrate surface and the final element of the projection system. The reservoir is formed by a seal member 12 positioned below and surrounding the final element of the projection system PL. Liquid is brought into the space below the projection system and within the seal member 12. The seal member 12 extends a little above the final element of the projection system and the liquid level rises above the final element so that a buffer of liquid is provided. The seal member 12 has an inner periphery that at the upper end, for example, closely conforms to the shape of the projection system or the final element thereof and may, e.g., be round. At the bottom, the inner periphery closely conforms to the shape of the image field, e.g., rectangular though this need not be the case.
The liquid is confined in the reservoir by a gas seal 16 between the bottom of the seal member 12 and the surface of the substrate W. The gas seal is formed by gas, e.g., air or synthetic air and in certain cases, N2 or another inert gas, provided under pressure via inlet 15 to the gap between seal member 12 and substrate and extracted via first outlet 14. The overpressure on the gas inlet 15, vacuum level on the first outlet 14 and geometry of the gap are arranged so that there is a high-velocity air flow inwards that confines the liquid.
In European Patent Application No. 03257072.3, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two stages for supporting the substrate. Leveling measurements are carried out with a stage at a first position, without immersion liquid, and exposure is carried out with a stage at a second position, where immersion liquid is present. Alternatively, the apparatus has only one stage.
According to the state of the art, the liquid supply system may be guided with respect to the substrate by way of an air bearing. The air bearing provides for a guiding of the liquid supply system with respect to the substrate, and for a distance between a surface of the substrate and the liquid supply system, or at least the reservoir thereof.
An alternative known in the art is to position the liquid supply system by way of an actuator instead of the guiding by an air bearing. The actuator is commonly driven by a control device providing for a positioning of the liquid supply system. In operation, the substrate is positioned such that its surface is kept in a focus plane of the projection system of the lithographic apparatus. Hence, according to the state of the art the liquid supply is positioned at a certain height with respect to the focus plane, to leave a predetermined gap between the liquid supply system (or at least the reservoir thereof) and the focus plane. As the substrate is positioned such that its surface coincides as good as possible with the focus plane, this will result in a distance between the surface of the substrate and the liquid supply system or at least the reservoir thereof, which is substantially equal to the gap between the focus plane and the liquid supply system (or reservoir thereof).
A problem associated with the positioning of the liquid supply system as described above occurs because of an unflatness of the substrate. To cope with unflatness of the substrate, the substrate table and positioning system thereof is according to the state of the art constructed to tilt the substrate table as to keep a part of the substrate which is to be illuminated at a certain moment locally in focus, thus locally coinciding as good as possible with the focus plane of the projection system. Dimensions of the liquid supply system are however significantly larger than a target portion of the substrate which is to be irradiated by way of the projection system. As a result thereof, a distance between an edge of the liquid supply system and the surface of the substrate may vary significantly, in practical implementations a variation in an order of magnitude of 30 micrometers having been observed. Such a variation in distance between the liquid supply system and the surface of the substrate may result in a crash of the liquid supply system against the substrate or may result in a too large distance causing leakage of the immersion liquid.
In addition to the above adverse effects, a further problem will occur at an edge of the substrate. In common state of the art implementations, the substrate, when being positioned on the substrate table, is surrounded by a structure which may comprise sensors, closing discs, etc. As explained above, a dimension of the liquid supply system may be significantly larger than a dimension of the target portion of the substrate, which is to be irradiated at a certain moment in time. Thus, when a portion of the substrate is to be irradiated, which is near an edge thereof, the liquid supply system will partly overlap with the structure(s) surrounding the substrate when positioned on the substrate table. A tolerance in the thickness of the substrate may thus result in a height difference between the surrounding structure and the surface of the substrate, hence resulting in a distance between the surrounding structure and the liquid supply system which may be larger or smaller than a distance between the target portion of the substrate which is to be irradiated and the liquid supply system. Similarly to the distance deviation as described above this may result in crashes in case of a distance which is too small or leakage of the immersion liquid in case the distance is too large.